A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible spectrum region of the electromagnetic spectrum.
A wide variety of technologically useful applications rely on photopolymers; for example, some enamel paint and depend on photopolymer formulation for proper hardening upon exposure to light. In some instances, an enamel can cure in a fraction of a second when exposed to light, as opposed to thermally cured enamels which can require half an hour or longer.
Changes in structural and chemical properties can be induced internally by chromophores that the polymer subunit already possesses, or externally by addition of photosensitivity molecules. Typically a photopolymer consists of a mixture of multifunctional monomers and oligomers in order to achieve the desired physical properties, and therefore a wide variety of monomers and oligomers have been developed that can polymerization in the presence of light either through internal or external initiation. Photopolymers undergo a process called curing, where oligomers are cross-linked upon exposure to light, forming what is known as a network polymer. The result of photo-curing is the formation of a thermoset network of polymers. One of the advantages of photo-curing is that it can be done selectively using high energy light sources, for example lasers, however, most systems are not readily activated by light, and in this case a photoinitiator is required. Photoinitiators are compounds that upon radiation of light decompose into reactive species that activate polymerization of specific on the oligomers.
There are two general routes for photoinitiation: free radical and ionic bond.
Ionic mechanism
In ionic curing processes, an ionic
photoinitiator is used to activate the
functional group of the
oligomers that are going to participate in cross-linking. Typically
polymerization is a very selective process and it is crucial that the
polymerization takes place only where it is desired to do so. In order to satisfy this, liquid neat oligomer can be doped with either
anionic or
cationic photoinitiators that will initiate polymerization only when radiated with
light.
, or functional groups, employed in cationic photopolymerization include:
styrene compounds,
enol ether, N-vinyl
,
lactones, lactams, cyclic
ethers, cyclic
, and cyclic
. The majority of ionic photoinitiators fall under the cationic class; anionic photoinitiators are considerably less investigated.
There are several classes of cationic initiators, including
onium compounds, organometallic compounds and
pyridinium salts.
As mentioned earlier, one of the drawbacks of the photoinitiators used for photopolymerization is that they tend to absorb in the short
ultraviolet.
Photosensitizers, or
, that absorb in a much longer wavelength region can be employed to excite the photoinitiators through an energy transfer.
Other modifications to these types of systems are
free radical assisted cationic polymerization. In this case, a free radical is formed from another species in solution that reacts with the photoinitiator in order to start polymerization. Although there are a diverse group of compounds activated by cationic photoinitiators, the compounds that find most industrial uses contain
, oxetanes, and vinyl ethers.
One of the advantages to using cationic photopolymerization is that once the polymerization has begun it is no longer sensitive to
oxygen and does not require an
Inert gas atmosphere to perform well.
- ;Photolysis
- :
\begin{matrix}{}\\
\ce{R'-R+X^- ->hv {R'-R+X-^\ast} -> {R^{.+}} + {R'^.} + X^- ->\ce{MH} + {^\bullet M_\mathit{m}R} -> {RM_\mathit{n}} + M_\mathit{m}R
Most composites that cure through radical chain growth contain a diverse mixture of oligomers and monomers with functionality that can range from 2-8 and molecular weights from 500 to 3000. In general, monomers with higher functionality result in a tighter crosslinking density of the finished material.
Typically these oligomers and monomers alone do not absorb sufficient energy for the commercial light sources used, therefore photoinitiators are included.
Free-radical photoinitiators
There are two types of free-radical photoinitators: A two component system where the radical is generated through
abstraction of a hydrogen atom from a donor compound (also called co-initiator), and a one-component system where two radicals are generated by
cleavage. Examples of each type of free-radical photoinitiator is shown below.
Benzophenone, xanthones, and quinones are examples of abstraction type photoinitiators, with common donor compounds being aliphatic amines. The resulting R• species from the donor compound becomes the initiator for the free radical polymerization process, while the radical resulting from the starting photoinitiator (benzophenone in the example shown above) is typically unreactive.
Benzoin ethers, Acetophenones, Benzoyl Oximes, and Acylphosphines are some examples of cleavage-type photoinitiators. Cleavage readily occurs for the species, giving two radicals upon absorption of light, and both radicals generated can typically initiate polymerization. Cleavage type photoinitiators do not require a co-initiator, such as aliphatic amines. This can be beneficial since amines are also effective chain transfer species. Chain-transfer processes reduce the chain length and ultimately the crosslink density of the resulting film.
Oligomers and monomers
The properties of a photocured material, such as flexibility, adhesion, and chemical resistance, are provided by the functionalized oligomers present in the photocurable composite. Oligomers are typically
epoxides,
Polyurethane,
polyethers, or
polyesters, each of which provide specific properties to the resulting material. Each of these oligomers are typically functionalized by an
acrylate. An example shown below is an epoxy oligomer that has been functionalized by
acrylic acid. Acrylated epoxies are useful as coatings on metallic substrates and result in glossy hard coatings. Acrylated urethane oligomers are typically abrasion resistant, tough, and flexible, making ideal coatings for floors, paper, printing plates, and packaging materials. Acrylated polyethers and polyesters result in very hard solvent resistant films, however, polyethers are prone to UV degradation and therefore are rarely used in UV curable material. Often formulations are composed of several types of oligomers to achieve the desirable properties for a material.
The monomers used in radiation curable systems help control the speed of cure, crosslink density, final surface properties of the film, and viscosity of the resin. Examples of monomers include styrene, N-Vinylpyrrolidone, and acrylates. Styrene is a low cost monomer and provides a fast cure, N-vinylpyrrolidone results in a material that is highly flexible when cured and has low toxicity, and acrylates are highly reactive, allowing for rapid cure rates, and are highly versatile with monomer functionality ranging from monofunctional to tetrafunctional. Like oligomers, several types of monomers can be employed to achieve the desired properties of the final material.
Applications
Photopolymerization has wide-ranging applications, from imaging to biomedical uses.
Dentistry
Dentistry is one field in which free radical photopolymers have found wide usage as adhesives, sealant composites, and protective coatings. These
are based on a camphorquinone
photoinitiator and a matrix containing
methacrylate with inorganic fillers such as
silicon dioxide. Resin cements are utilized in
luting agent cast
ceramic, full
porcelain, and veneer restorations that are thin or translucent, which permits visible light penetration in order to polymerize the cement. Light-activated cements may be radiolucent and are usually provided in various shades since they are utilized in esthetically demanding situations.
[ DIS55]
Conventional , and xenon Arc lamp are currently used in clinical practice. A new technological approach for curing light-activated oral using a light curing unit (LCU) is based on blue light-emitting diodes (LED). The main benefits of LED LCU technology are the long lifetime of LED LCUs (several thousand hours), no need for filters or a cooling fan, and virtually no decrease of light output over the lifetime of the unit, resulting in consistent and high quality curing. Simple depth of cure experiments on cured with LED technology show promising results.
Medical uses
Photocure adhesives are also used in the production of
catheters,
,
, medical filters, and blood analysis sensors.
Photopolymers have also been explored for uses in drug delivery, tissue engineering and cell encapsulation systems.
Photopolymerization processes for these applications are being developed to be carried out
in vivo or
ex vivo.
In vivo photopolymerization would provide the advantages of production and implantation with minimal invasive surgery.
Ex vivo photopolymerization would allow for fabrication of complex matrices and versatility of formulation. Although photopolymers show promise for a wide range of new biomedical applications, biocompatibility with photopolymeric materials must still be addressed and developed.
3D printing
Stereolithography,
digital imaging, and 3D inkjet printing are just a few 3D printing technologies that make use of photopolymerization pathways. 3D printing usually utilizes CAD-CAM software, which creates a 3D computer model to be translated into a 3D plastic object. The image is cut in slices; each slice is then reconstructed through radiation curing of the liquid
polymer, converting the image into a solid object. Photopolymers used in 3D imaging processes require sufficient cross-linking and should ideally be designed to have minimal volume shrinkage upon
polymerization in order to avoid distortion of the solid object. Common monomers utilized for 3D imaging include multifunctional
and
, often combined with a non-polymeric component in order to reduce volume shrinkage.
A competing composite mixture of epoxide resins with cationic photoinitiators is becoming increasingly used since their volume shrinkage upon ring-opening polymerization is significantly below those of acrylates and methacrylates. Free-radical and cationic polymerizations composed of both epoxide and acrylate monomers have also been employed, gaining the high rate of polymerization from the acrylic monomer, and better mechanical properties from the epoxy matrix.
Photoresists
are coatings, or
, that are deposited on a surface and are designed to change properties upon irradiation of
light. These changes either
polymerization the liquid oligomers into insoluble cross-linked network polymers or decompose the already solid polymers into liquid products. Polymers that form networks during
polymerization are referred to as
photoresist. Conversely,
that decompose during photopolymerization are referred to as
photoresist. Both positive and negative resists have found many applications including the design and production of micro-fabricated chips. The ability to pattern the resist using a focused light source has driven the field of
photolithography.
Negative resists
As mentioned,
photoresist are photopolymers that become insoluble upon exposure to radiation. They have found a variety of commercial applications, especially in the area of designing and printing small chips for electronics. A characteristic found in most negative tone resists is the presence of
functional group branches on the
used. Radiation of the polymers in the presence of an
photoinitiator results in the formation of a chemically resistant network polymer. A common
functional group used in negative resists is
epoxy functional groups. An example of a widely used
polymer of this class is SU-8. SU-8 was one of the first polymers used in this field, and found applications in wire board printing.
In the presence of a
cationic photoinitiator photopolymer, SU-8 forms networks with other polymers in solution. Basic scheme shown below.
SU-8 is an example of an intramolecular polymerization forming a matrix of cross-linked material. Negative resists can also be made using co-polymerization. In the event that two different , or , are in solution with multiple functional group, it is possible for the two to polymerize and form a less soluble polymer.
Manufacturers also use light curing systems in OEM assembly applications such as specialty electronics or medical device applications.
Positive resists
Exposure of a
photoresist to radiation changes the chemical structure such that it becomes a liquid or more soluble. These changes in chemical structure are often rooted in the cleavage of specific
cross-link in the
polymer. Once irradiated, the "decomposed" polymers can be washed away using a developer
solvent leaving behind the polymer that was not exposed to light. This type of technology allows the production of very fine stencils for applications such as
microelectronics.
In order to have these types of qualities, positive resists utilize polymers with
labile linkers in their back bone that can be cleaved upon irradiation, or use a
photoinitiator to
hydrolyze bonds in the polymer. A polymer that decomposes upon irradiation to a liquid or more soluble product is referred to as a
photoresist. Common
that can be hydrolyzed by a photo-generated acid catalyst include
and
.
Fine printing
Photopolymers can be used to generate printing plates, which are then pressed onto paper-like
metal type.
This is often used in modern fine printing to achieve the effect of
paper embossing (or the more subtly three-dimensional effect of letterpress printing) from designs created on a computer without needing to engrave designs into metal or cast metal type. It is often used for business cards.
Repairing leaks
Industrial facilities are utilizing light-activated resin as a sealant for leaks and cracks. Some light-activated resins have unique properties that make them ideal as a pipe repair product. These resins cure rapidly on any wet or dry surface.
Fishing
Light-activated resins recently gained a foothold with fly tiers as a way to create custom flies in a short period of time, with very little clean up involved.
Floor refinishing
Light-activated resins have found a place in floor refinishing applications, offering an instant return to service not available with any other chemical due to the need to cure at ambient temperatures. Because of application constraints, these coatings are exclusively UV cured with portable equipment containing high intensity discharge lamps. Such UV coatings are now commercially available for a variety of substrates, such as wood, vinyl composition tile and concrete, replacing traditional polyurethanes for wood refinishing and low durability acrylics for VCT.
Environment Pollution
Washing the polymer plates after they have been exposed to ultra-violet light may result in monomers entering the sewer system, eventually adding to the plastic content of the oceans. Current water purification installations are not able to remove monomer molecules from sewer water. Some monomers, such as
styrene, are toxic or
carcinogenic.
